Widely tunable swept source

ABSTRACT

A high-speed, single-mode, high power, reliable and manufacturable wavelength-tunable light source operative to emit wavelength tunable radiation over a wavelength range contained in a wavelength span between about 950 nm and about 1150 nm, including a vertical cavity laser (VCL), the VCL having a gain region with at least one compressively strained quantum well containing Indium, Gallium, and Arsenic.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/793,730 filed on Mar. 15, 2013, currently pending.The disclosure of U.S. Provisional Patent Application 61/793,730 ishereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made under NIH grant R44CA101067 and R44EY022864. TheU.S. government has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates generally to optical coherence tomography andtunable lasers.

BACKGROUND

Optical coherence tomography (OCT) is a technique for high-resolutiondepth profiling of a sample below the sample surface. In recent years,swept source optical coherence tomography (SSOCT) systems havedemonstrated superior imaging speed, imaging range, and image quality.The key technology element of SSOCT systems is the wavelength sweptlaser source. The MEMS-tunable vertical cavity laser (MEMS-VCL) hasproven to be an important key wavelength-swept source for SS-OCT at 1300nm and 1050 nm, as described, for example in (I. Grulkowski, J Liu, B.Potsaid, V. Jayaraman, C. D. Lu, J. Jiang, A. E. Cable, J. S. Duker, andJ. G. Fujimoto, “Retinal, anterior segment, and full-eye imaging usingultra-high speed swept source OCT with vertical cavity surface-emittinglasers,” Biomedical Optics Express, vol. 3, no. 11, pp. 2733-2751). ForSS-OCT systems to be commercially viable, swept sources based on 1050 nmVCLs must provide substantially single longitudinal, transverse andpolarization mode operation over a wide tuning range, be swept athundreds of kHz rates for hundreds of billions of cycles, providesufficient output power for SSOCT imaging, and be manufacturable andlong-term reliable.

From the foregoing, it is clear that what is required is a MEMS-VCL at1050 nm that meets tuning range, speed, coherence length, and outputpower requirements of SS-OCT systems, and is both manufacturable andlong-term reliable.

SUMMARY OF THE INVENTION

The present invention provides several preferred and alternatemanufacturable and reliable embodiments of a high-speed, single-mode,high power, reliable and manufacturable swept laser source based arounda tunable 1050 nm VCL.

One embodiment provides a wavelength-tunable light source operative toemit wavelength tunable radiation over a wavelength range contained in awavelength span between about 950 nm and about 1150 nm, the wavelengthtunable light source including a vertical cavity laser (VCL), the VCLhaving a gain region with at least one compressively strained quantumwell containing Indium, Gallium, and Arsenic, the vertical cavity laserfurther comprising a first portion including a first mirror, a secondportion including a second mirror attached to a mechanical structureincluding a flexible membrane with a support structure, an adjustableairgap between the second portion and the first portion, a first meansfor injecting electrons and holes into the gain region, a second meansfor adjusting the airgap, and a third means for obtaining substantiallysingle longitudinal and transverse mode operation over the wavelengthtuning range, wherein a peak room-temperature photoluminescencewavelength of the gain region is more than about 20 nm shorter than amaximum operating wavelength of the tunable laser.

Another embodiment provides wavelength-tunable light source operative toemit wavelength tunable radiation over a wavelength range contained in awavelength span between about 950 nm and about 1150 nm, the wavelengthtunable light source including a vertical cavity laser (VCL), the VCLhaving a gain region with at least one compressively strained quantumwell containing Indium, Gallium, and Arsenic, the vertical cavity laserfurther comprising a first portion including a first mirror, a secondportion including a second mirror attached to a mechanical structureincluding a flexible membrane with a support structure, an adjustableairgap between the second portion and the first portion, a first meansfor injecting electrons and holes into the gain region, a second meansfor adjusting the airgap, and a vacuum environment surrounding thevertical cavity laser.

Another embodiment provides a wavelength-tunable light source operativeto emit wavelength tunable radiation over a wavelength range containedin a wavelength span between about 950 nm and about 1150 nm, thewavelength tunable light source including a vertical cavity laser (VCL),the VCL having a gain region with at least one compressively strainedquantum well containing Indium, Gallium, and Arsenic, the verticalcavity laser further comprising a first portion including a firstmirror, a second portion including a second mirror attached to amechanical structure including a flexible membrane with a supportstructure, an adjustable airgap between the second portion and the firstportion, a first means for electrical injection of electrons and holesinto the gain region, the first means including a tunnel junction, asecond means for adjusting the airgap, and a third means for obtainingsubstantially single longitudinal and transverse mode operation over thewavelength tuning range.

Another embodiment provides a wavelength-tunable light source operativeto emit wavelength tunable radiation over a wavelength range containedin a wavelength span between about 950 nm and about 1150 nm, thewavelength tunable light source including a vertical cavity laser (VCL),the VCL having a gain region with at least one compressively strainedquantum well containing Indium, Gallium, and Arsenic, the verticalcavity laser further comprising a first portion including a firstmirror, a second portion including a second mirror attached to amechanical structure including a flexible membrane with a supportstructure, an adjustable airgap between the second portion and the firstportion, a first means for injecting electrons and holes into the gainregion, a second means for adjusting the airgap, and a third means forobtaining substantially single longitudinal and transverse modeoperation over the wavelength tuning range, said wavelength tunablelight source further comprising a semiconductor optical amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optically pumped MEMS-VCL device according to anembodiment.

FIG. 2 shows the top mirror spectrum of a MEMS-VCL according to anembodiment.

FIG. 3 shows a device with WDM separation according to an embodiment.

FIG. 4 shows a bottom-emission device according to an embodiment.

FIG. 5 shows a device with off-axis pumping according to an embodiment.

FIG. 6 shows a conduction band and VCL standing wave profile of a deviceaccording to an embodiment.

FIG. 7 shows a conduction band and VCL standing wave profile of a deviceaccording to another embodiment.

FIG. 8 shows a conduction band and VCL standing wave profile of a deviceaccording to another embodiment.

FIG. 9 illustrate the definition of FSR and desired zero biaswavelength.

FIG. 10 shows the static and dynamic tuning range of a device accordingto an embodiment.

FIG. 11 is a schematic diagram of an electrical pumped MEMS-VCL deviceaccording to an embodiment.

FIG. 12 is a schematic diagram of an electrical pumped MEMS-VCL withburied tunnel junction device according to an embodiment.

FIG. 13 is a schematic diagram of an electrical pumped MEMS-VCL withevaporated bottom mirror according to another embodiment.

FIG. 14 shows the pre and post-amplified VCL spectra of a deviceaccording to an embodiment.

FIG. 15 shows a ridge waveguide semiconductor optical amplifier deviceaccording to an embodiment.

FIG. 16 shows the configuration of a device, the drive current,wavelength trajectory and multiplexed output versus time according to anembodiment.

DETAILED DESCRIPTION OF PREFERRED AND ALTERNATE EMBODIMENTS

FIG. 1 shows a schematic of a preferred embodiment of a highperformance, manufacturable, and reliable wavelength swept sourceaccording the present invention. The design of both the tuning mechanismand the gain medium will enable a mean time to failure of well over 1000hours, where failure is defined as the point when either achievabletuning range or output power drops by 10% or more. For someapplications, a shorter mean time to failure, such as 100 hours may besufficient. This source emits wavelength-swept radiation over awavelength tuning range contained in a range of about 950-1150 nm, withan average output power and a peak power wavelength. An optically pumpedtunable MEMS-VCL includes a fixed first portion 110 and a movable secondportion 100. Although the preferred tuning mechanism shown is aMEMS-tuning mechanism other tuning mechanisms such as electro-thermalactuation relying on an expanded membrane using resistive heating arepossible. The first portion includes an InGaAs MQW gain region 120 withat least one and ideally 3 compressively strained indium galliumarsenide (InGaAs) quantum wells with GaAs barriers, which will absorb apump radiation from an optically pumped laser source 125. The InGaAsquantum wells can comprise 2 quantum states for enhanced gain-bandwidth.The optically pumped laser source 125 has a wavelength in a range ofabout 750-870 nm wavelength, preferably at about 850 nm, and serves toinject electrons and holes into the gain region 120 to enable lasing.The first portion 110 also includes a bottom mirror 130, which is afully oxidized GaAs/AlxOy mirror, formed by lateral oxidation of aGaAs/AlAs alternating stack. The lower refractive index portion of thismirror can be also formed by oxidation of Al(x)Ga(1−x)As, with xpreferably >0.9. The high index portion also need not be GaAs and can beAlGaAs. The movable second portion 100 includes a top suspendeddielectric mirror 140, separated by an airgap 150 from the first portion110. An anti-reflection (AR) coating 160 at the interface between thefirst portion 110 and the airgap 150 suppresses unwanted reflections andwidens laser tuning range. The dielectric mirror 140 is ideally formedby a quarter wave stack of silicon dioxide (SiO2) and tantalum pentoxide(Ta2O5), although the Ta2O5 could be replaced by an oxide of niobium,hafnium, titanium, by silicon, or by a number of other high indexmaterials well-known to those skilled in the art of VCSEL design andfabrication. The dielectric mirror is also curved to provide ahalf-symmetric cavity, which promotes high power single-mode operation.The dielectric mirror sits on a membrane 170, which is preferablysilicon nitride, and is supported on its ends by a support structure185, which is preferably germanium. This support structure is ideallythe same material used as a sacrificial layer to undercut the membrane170. Other candidate materials for the support structure includesilicon, polyimide, photoresist, or SU-8. The use of a silicon nitridemembrane provides an extremely reliable membrane that can survive morethan a trillion cycles of flexure, as required in many high-speed SS-OCTsystems. The ideal support structure is one which can be undercut by adry gas such as xenon diflouride or oxygen, instead of by a wet chemicaletch. The undercut gas should not substantially etch other elements ofthe VCL structure. Use of a wet chemical etched sacrificial layernecessitates critical point drying, which complicates device processing,dicing, and packaging. For example, it is preferable to dice a processedwafer of VCLs into individual VCLs before undercutting the sacrificiallayer since this enables the structure to remain robust for handling.Undercut of a sacrificial layer at the die level is very difficult ifcritical point drying is required.

The silicon nitride membrane 170 is ideally integral with the dielectricmirror, and actually forms the first layer of the dielectric mirror. Inaddition, a lateral extent 165 of the suspended dielectric mirror 140 issmaller than a lateral extent 155 of the membrane 170. This reduces themass of the MEMS actuator, and increases the resonant frequency. Thesilicon nitride thickness is ideally an odd number of quarterwavelengths at a center of the wavelength range, preferably ¾wavelengths thick. A tunable VCL output 180 (1060 nm tunable emission)is amplified by a semiconductor optical amplifier 190 to create a highpower wavelength tunable radiation 195 which can be used for a varietyof applications, such as optical coherence tomography and spectroscopy.Tuning of the wavelength is accomplished by adjusting the airgap 150,ideally by applying a voltage between electrodes in the MEMS structureof FIG. 1. A well-designed structure as in FIG. 1 requires a maximumtuning voltage less than about 100V.

The mirror choices of the VCL in FIG. 1 promote wide tuning due to widemirror bandwidth. Other mirror combinations are possible, such as usinga more standard Al(x1)Ga(1−x1)As/Al(x2)Ga(1−x2)As semiconductor mirrorwith x1 and x2 between 0 and 1, instead of the fully oxidized mirror.Ideally the output suspended mirror is designed to have a reflectivitybetween 99.0 and 99.9% over the tuning range, and the fixed mirroris >99.9% reflectivity. Another less preferred mirror implementation isa high contrast grating (HCG) for the suspended mirror.

Efficient optical coupling of the optical pump source 125 to the VCL inFIG. 1 requires that the top dielectric mirror 140 have a spectrum thatis transparent at the pump wavelength. An example spectrum is shown inFIG. 2. The mirror design consists of 10.25 periods of a SiO2/Ta2O5quarter wave stack on a ¾ wavelength SiN membrane, with the top layerbeing a ⅛ wave layer of SiO2. This ⅛ wave layer flattens the spectrumnear the 850 nm pump wavelength.

One necessary requirement of the laser source in FIG. 1 is theseparation of pump light from the desired emission in the 950-1150 nmrange. This can be accomplished by a WDM coupler as in FIG. 3, where aninput fiber 200 brings in the 850 nm pump light to a WDM coupler 220,and an output fiber 210 takes out the tunable 1050 nm emission from thetunable VCL 240. An optical fiber 230 guides both incoming 850 nm pumplight to and outgoing 1050 nm VCL emission from the tunable VCL 240sitting on a GaAs substrate 250. In an alternate separation method inFIG. 4, the VCL 240 is configured to be bottom emitting, and the GaAssubstrate 250 absorbs the 850 nm pump radiation while passing only thetunable 1050 nm VCL emission. In yet another embodiment in FIG. 5, thetunable VCL 240 is pumped along an axis different from an optical axis260 of the tunable VCL.

A number of features of the structure of FIG. 1 promote low-noisesingle-mode operation. First, use of a single transverse andlongitudinal mode pump laser promotes both single transverse modeoperation and low relative intensity noise. Careful alignment of thepump beam to a lowest order transverse mode position of the VCL is alsocritical to ensuring good suppression of higher order transverse modesideally to better than 45 dB. This suppression is important to maintainlong coherence length for SSOCT imaging and avoid spurious image lines.Proper control of the half-symmetric cavity curvature in a range ofabout 0.5-3 mm with an airgap in a range of about 0.7-1.8 um alsopromotes single transverse mode operation. Single longitudinal operationis guaranteed by staying away from the edges of the wrap-around pointnear the edges of a free-spectral range (FSR) of the cavity. FSR isdiscussed further below and described with the aid of FIG. 9. Ideallythe tuning range of the laser should be less than about 95% of the FSRto promote single longitudinal mode operation.

FIG. 1 also shows a metal layer 198 at the back side of a GaAs substrate175. The metal layer is not needed for electrical contact, but promoteslow backside reflection, if the metal is titanium, chromium, orplatinum. Low backside reflection is necessary to enable tuning acrossthe tuning range with less than about 1% periodic ripple on the outputpower spectrum. The ripple is caused by a substrate reflection going inand out of phase with the bottom mirror 130 as the laser is tuned. Thisripple is further reduced by lapping the backside of the substrate 175with a lapping paper or solution with a grit size in a range of about 30um to about 120 um. Also, reducing the ripple is accomplished byincreasing the reflectivity of the GaAs—AlxOy mirror. If the theoreticallossless mirror reflectivity is increased above about 99.95%, substratereflections become less severe though backside lapping is usually stillnecessary. The theoretical lossless reflectivity means the calculatedreflectivity assuming zero loss in the mirror layers. This reflectivitycan be achieved through the use of 6 or more periods of the GaAs—AlxOymirror. The fully oxidized mirror also promotes operation in a constantpolarization state due to the incorporation of anisotropic stress.

A number of additional features of the preferred embodiment in FIG. 1promote high performance, reliability, and manufacturability. First, thegain region is designed such that the quantum wells have a roomtemperature photoluminescence (RTPL) peak wavelength that issubstantially shorter than a maximum operating wavelength of the laser.For example if the wavelength tuning range of the laser in FIG. 1 is1000-1100 nm, then the RTPL peak can be as short as 1020 nm. Thisreduces the required strain in the quantum wells and promotesreliability. The quantum well strain should be in a range of about1-1.8%. In general, placing the RTPL peak at least 20 nm shorter than amaximum operating wavelength of the laser enhances reliability of thedevice, and the shorter the PL wavelength, the more reliable the device.

The tuning range of the wavelength swept source shown in FIG. 1, and insubsequent electrically pumped embodiments in FIGS. 11-13, is largelydetermined by a tuning range of the VCL. The VCL tuning range ismaximized by a thin cavity having a large free spectral range (FSR),since the maximum tuning range is the FSR. The FSR is calculated asΔλ=λ²/2L_(eff), where L_(eff) is the effective cavity length in air,accounting for refractive index and penetration into the mirrors of thedevice. This is the wavelength spacing between longitudinal cavitymodes, as shown in FIG. 9. For use in ophthalmic SSOCT, the FSR isideally greater than 100 nm for high resolution imaging with ˜100 nmtuning range, but FSR exceeding 70 nm or 40 nm can also provide usefuldevices for lower resolution imaging at for example longer imagingranges.

Also shown in FIG. 9 is a preferred zero bias wavelength 370, shown indashes. In a MEMS structure, the ideal position of the zero biaswavelength is to left edge 360 of one FSR, since application of a smallbias will cause the mode to tune shorter and wrap around to the longeredge of the FSR at 380. This enables accessing a full FSR in tuning Ifthe zero bias position 370 were closer to the right edge at 380, thenfull tuning would be problematic to achieve, since application ofvoltage in a MEMS structure can generally only shorten the airgap. Ifthe zero-bias wavelength is near the right edge at 380, thenprohibitively high voltage would be required to wrap around to the nextFSR. Such voltage would likely exceed a snapdown voltage of the device.Note that it is possible through inertial effects to bounce past thezero bias wavelength of the device under dynamic repetitively sweptoperation, and access wavelengths longer than the zero bias wavelengthin this manner. This is not preferable for SS-OCT imaging, however.

Accessing full tuning range also requires proper design of the initialairgap. Since covering one FSR requires a membrane deflection equal toone half a wavelength or about 0.53 um near 1060 nm, and because staticdeflection of more than ⅓ the total gap is prohibited by snapdown in aMEMS structure, the ideal airgap should exceed 3 half-wavelengths orabout 1.6 um. In practice for SSOCT systems, tuning is done underdynamic operation in which it is possible to exceed snapdown. Thus a gapin the range of 2-3 half wavelengths is sufficient to guarantee fulltuning under dynamic operation at several hundred kilohertz withoutsnapdown problems. FIG. 10 shows an example of static and dynamic tuningin a 1050 nm VCL, where a 100 nm dynamic tuning range exceeds a 90 nmstatic tuning range.

The placement of quantum wells inside the cavity, relative to an opticalstanding wave in the laser cavity also promotes performance andreliability. FIGS. 6-8 illustrate 3 preferred designs for quantum wellplacement in the structure of FIG. 1. The horizontal axis in thesefigures is distance along an optical propagation axis of the VCL cavity.The top of each figure represents the conduction band of material in thestructure illustrating quantum well and absorber locations. The bottomof each figure represents a standing wave of intra-cavity radiation ineach structure. In FIG. 6, three quantum wells of a multi-quantum wellregion 300 are aligned with 3 separate maxima of a standing wave pattern310 of the optical cavity. In this periodic gain structure, gain is notonly enhanced relative to quantum well placement away from standing wavepeaks, but the wells are sufficiently separated that they are de-coupledwith respect to strain accumulation. This means that the total strainthickness product of the quantum wells can exceed the normalstrain-thickness product limit of 200 Angstrom-percent. Also shown is anabsorption region 315 with thickness of about 0.45 microns, which leadsto absorption of more than 40% of the 850 nm pump power. Most of theabsorption occurs in the GaAs barriers 305 of the quantum wells, andonly a minority of the absorption happens in the wells. Allphotogenerated electrons and holes diffuse into the quantum wells 300,so absorbed pump light is efficiently converted to tunable VCL light.

Another preferred MQW embodiment is shown by FIG. 7. Here, all threequantum wells of a MQW region 320 are aligned with a single standingwave peak of a standing wave pattern 330 inside the VCL optical cavity.In this case tensile-strained GaAsP strain-compensated barriers areprovided to compensate the compressively strained InGaAs quantum wells.The advantage of FIG. 7 is that the cavity thickness can be reduced,leading to a larger cavity free spectral range and wider tuning Thedisadvantage is a reduced thickness absorption region 335, having about0.1 um thickness, causing an increase in required pump power. Heresignificant absorption happens in both wells and barriers of thestructure.

A third preferred placement is shown in FIG. 8, where 2 sets of 2quantum wells in an MQW region 340 are aligned with 2 standing wavepeaks of an optical standing wave pattern 350. Here an absorber 355 ofthickness around 0.3 um is employed, leading to a compromise betweenpump absorption and wide free spectral range. This structure alsoemploys tensile-strained GaAsP barriers.

Reliable operation of the VCL shown in FIG. 1 is also promoted bylimiting a pump power presented to the VCL. An absorbed power of 0.3 to30 mW is preferable to maintain reliable operation, and an absorbed pumppower less than about 15 mW is ideal.

FIG. 11 illustrates another preferred embodiment of a wavelength-sweptsource according to the present invention. In this figure electrons andholes are provided to the gain region through an electrical pump currentinstead of an optical pump. A bottom MEMs contact 400 also serves as thetop contact for injection of current into the quantum wells. The contact400 injects electrons into an n-region 410, which are converted to holesin a p-type region 430 through an n+/p+ tunnel junction 420, which ispreferably n+GaAs/p+GaAs. Contact to the n-layer 410 enables efficientlateral spreading of the current and promotes single-mode operation. Thep-type region 430 contains a current constriction oxide aperture 440which is formed by lateral oxidation of AlGaAs. Carriers are injectedfrom the region 430 through the current aperture 440 into the gainregion comprised of an InGaAs multiquantum well (MQW) 450 with GaAsPbarriers 460. Oppositely charged carriers are injected into the MQW froma substrate contact 470 through the GaAs substrate 480 and GaAs cladding490, around a fully oxidized mirror 500 comprising insulating AlxOylayers. In this structure, the tunnel junction 420 is preferably placedat a node in the optical standing wave pattern inside the cavity, toreduce sensitivity to free carrier losses in the highly doped region. Inaddition, placement of the oxide aperture 440 near a node promotessingle-mode operation, as it reduces the waveguide confinement of theoxide.

Polarization control of this structure is partially provided byanisotropic stress of the fully oxidized mirror, but furtherpolarization selection can be provided by incorporation of one or morenanowires 485 or a sub-wavelength grating 475 at the top or bottom ofthe of the suspended DBR. These approaches can also be employed inoptically pumped structures such as in FIG. 1. In an electrically pumpedstructure, previous workers have employed asymmetric current injectionin fixed wavelength structures, which can also be employed inelectrically pumped tunable VCLs here.

An alternate preferred electrically pumped embodiment is shown in FIG.12, in which constriction is provided by a buried tunnel junction 510 oflimited lateral extent instead of the oxide aperture 440 in FIG. 11. Theburied tunnel junction has the advantage of being lithographicallydefined and therefore more controllable than the oxide aperture 440, thelatter defined by oxidation time and temperature, but has thedisadvantage of requiring a regrowth step to bury the tunnel junction.Ideally, the buried tunnel junction 510 is aligned with a standing wavenode to minimize free carrier losses.

FIG. 13 shows a third preferred embodiment of an electrically pumpedVCL, in which the fully oxidized mirror 500 is replaced by an evaporatedmirror 520 evaporated through a substrate via hole 530. A preferredembodiment of the evaporated mirror is a quarter wave stack of aluminumfluoride and Zinc Sulfide, terminated by a metal layer such as gold.Other high index contrast wide bandwidth evaporated stacks include otherfluorine compounds such as magnesium fluoride as the low index layer.

The electrically and optically pumped embodiments shown in FIGS. 1,11-13 all include a semiconductor optical amplifier (SOA) to amplify atunable radiation emitted by the VCL. The SOA is not required for allapplications, but is desired in most swept source OCT applications whichrequire high average power across the wavelength range. The design ofthis SOA is critical to performance, reliability and manufacturability.FIG. 14 illustrates how SOA gain saturation can provide an improvedfull-width half maximum (FWHM) 600 of post-amplified VCL radiation,relative to a FWHM 610 of pre-amplified VCL emission. The peakwavelength of the spectrum is also shifted from a pre-amplified peakwavelength 620 to a post-amplified peak wavelength 630. This shift canalso be advantageous for many applications.

The preferred SOA uses compressively strained InGaAs quantum wells witha strain level in a range of about 1-1.8%. The preferred SOA is alsopolarization sensitive, amplifying only one polarization. An alternateless preferred implementation uses an InGaAs quantum dot amplifier. Onepreferred design uses a ridge waveguide design with 2 InGaAs quantumwells with tensile strained GaAsP barriers in a waveguide having a FWHMvertical beam divergence less than 25 degrees, as shown in the exampledivergence angles of FIG. 15. The divergence angle is defined as theangle of the amplified spontaneous emission emitted by the amplifierwhen operated as a superluminescent diode. Low divergence angle improvesfiber-coupling efficiency and chip-to-chip gain of the SOA. The gain isalso aided by the use of 2 quantum wells. For some applications, asingle quantum well can be employed providing less gain, and with onequantum well strain compensation is not necessary. In all strained QWSOAs using InGaAs, the preferred well width is in a range of 5-10 nm.Reliability is improved by thinner quantum wells, but a second quantumstate appearing in the well in a width range of about 8-10 nm improvesgain bandwidth. Reliability is also improved by minimizing a roomtemperature photoluminescence (RTPL) wavelength of the quantum wells,with a range of about 1050-1085 nm desirable for good reliability andhigh gain in a wavelength range from around 980 nm to around 1120 nm.

Another factor in InGaAs quantum well SOA design is providing high gainwithout sacrificing reliability. This requires operating in the properregime of device length and operating current. Ideally the device lengthshould be between about 1.2 mm and about 2.0 mm, and the drive currentshould be between about 200 mA and about 700 mA. This enables outputpowers of 10-30 mW with input powers of 0.3-3 mW.

Another consideration in the design of the wavelength swept sources inFIGS. 1, 11-13 above is the speed of tuning A number of design choicesenhance tuning speed in a MEMS tunable structure. One is the use ofstress in the SiN membrane 170. Tensile stresses of >100 MPascal arepreferred, and give a mechanical resonance that is substantiallyincreased relative to a zero-stress film. Tensile stress>400 Mpa lead tovery high resonances approaching 500 kHz and higher depending on thegeometry. Compressive stress could also be employed, but tensile stressenables better wavelength control. Stressed membranes also enablecontrol of zero bias wavelength through the amount of sacrificial layerundercutting. The zero bias wavelength is a function of the amount ofsacrificial undercut, which enables zero-bias wavelength adjustmentduring fabrication. Combining increased stress with small geometries canincrease the MEMS mechanical resonance beyond 1 MHz. Important resonanceregimes are 10 kHz, 100 kHz, 200 kHz, 400 kHz, and 1 MHz. All of thesecan be achieved by control of geometry and stress. The structure of FIG.1 illustrates 3 supporting arms, but 2, 4, or 8 are desirable numbers ofsupporting arms for various applications. The basic geometry of acentral plate with multiple supporting arms can be changed to aperforated diaphragm in the limit where the arm length goes to zero andperforations are introduced in the central plate.

In addition to mechanical resonance, the frequency response of the MEMSstructure can be flattened by using squeeze-film damping introduced byviscous air. This can be controlled by adjustment of background gas orpressure, and by control of the central plate and arm areas. In general,increased pressure, heavier gases, and wider arms/plates increasedamping and flatten frequency response. Using these parameters, nearcritically damped operation can be achieved. In some applications,highly underdamped resonant operation in vacuum is desirable as thisreduces required voltage to typically less than 10V for full tuningVacuum environments can be provided by evacuated butterfly or transistoroutline (TO) packages.

The speed of the optically and electrically pumped swept sources inFIGS. 1, 11-13 can also be increased by incorporation of one or moredelay lines and multiplexing of time delayed outputs. An example of thisfor one delay line which doubles sweep rate is shown in FIG. 16. Anelectrically pumped VCL output with a first wavelength repetition period700 shown in FIG. 16C is split to a first optical path 710 and a secondtime-delayed optical path 720, the path 720 delayed half a repetitionperiod, as shown in FIG. 16A. The wavelength is scannedbi-directionally, as shown in FIG. 16C, but the VCSEL is turned offduring the backward sweep by turning off drive current 730 as shown inFIG. 16B. The multiplexed output containing both the VCL output anddelayed output is shown in FIG. 16D. The copy is inserted during theblanked backward sweep, resulting in a final unidirectional scan with arepetition period 740 that is half the repetition period 700 (twice therepetition frequency), with nearly 100% duty cycle. The technique ofFIG. 16 can be extended to N delay lines with the final multiplexedsweep having a repetition frequency that is N times the repetitionfrequency of the original VCL sweep.

The wavelength swept light source described here has application inswept source OCT (SSOCT) imaging of the human eye. The single modeoperation guarantees long coherence length exceeding 100 mm, enablingwhole eye imaging including both the anterior eye and retinal layers.

Though the invention described here has been focused on the 950-1150 nmwindow, many of the design principles presented can be applied to otherwavelength regimes. In addition, many design principles described withrespect to electrically pumped devices can be applied to opticallypumped devices and vice versa. While this invention has beenparticularly shown and described with references to preferred andalternate embodiments thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:
 1. A wavelength-tunable light source operative toemit wavelength tunable radiation over a wavelength range contained in awavelength span between about 950 nm and about 1150 nm, said wavelengthtunable light source comprising a vertical cavity laser (VCL), said VCLhaving a gain region with at least one compressively strained quantumwell containing Indium, Gallium, and Arsenic, said vertical cavity laserfurther comprising a first portion including a first mirror, a secondportion including a second mirror attached to a mechanical structureincluding a flexible membrane with a support structure, an adjustableairgap between said second portion and said first portion, a first meansfor injecting electrons and holes into said gain region, a second meansfor adjusting said airgap, and a third means for obtaining substantiallysingle longitudinal and transverse mode operation over said wavelengthtuning range, wherein a peak room-temperature photoluminescencewavelength of said gain region is more than about 20 nm shorter than amaximum operating wavelength of said tunable laser.
 2. The wavelengthtunable light source of claim 1, wherein a peak room-temperaturephotoluminescence wavelength of said gain region is at more than about50 nm shorter than a maximum operating wavelength of said tunable laser.3. The wavelength tunable light source of claim 1, wherein a peakroom-temperature photoluminescence wavelength of said gain region ismore than about 70 nm shorter than a maximum operating wavelength ofsaid tunable laser.
 4. The wavelength tunable light source of claim 1,wherein said first mirror comprises an alternating stack including afirst Al(x1)Ga(1−x1)As layer and a second Al(x2)Ga(1−x2)As layer, wherex1 and x2 are in a range of 0-1.
 5. The wavelength tunable light sourceof claim 1, where said first mirror comprises an alternating stackincluding a first material containing aluminum and oxygen, and a secondmaterial that is Al(x)Ga(1−x)As, where x is in a range of 0-1.
 6. Awavelength-tunable light source operative to emit wavelength tunableradiation over a wavelength range contained in a wavelength span betweenabout 950 nm and about 1150 nm, said wavelength tunable light sourcecomprising a vertical cavity laser (VCL), said VCL having a VCL gainregion with at least one compressively strained quantum well containingIndium, Gallium, and Arsenic, said vertical cavity laser furthercomprising a first portion including a first mirror, a second portionincluding a second mirror attached to a mechanical structure including aflexible membrane with a support structure, an adjustable airgap betweensaid second portion and said first portion, a first means for injectingelectrons and holes into said gain region, a second means for adjustingsaid airgap, a third means for obtaining substantially singlelongitudinal and transverse mode operation over said wavelength tuningrange, said wavelength-tunable light source further comprising asemiconductor optical amplifier (SOA) comprising at least one SOAquantum well including Indium, Gallium, and Arsenic, said at least oneSOA quantum well having a room temperature photoluminescence wavelengthin a range of about 1050 nm-1085 nm.
 7. The wavelength tunable lightsource of claim 6, wherein said semiconductor optical amplifier includesexactly one compressively strained SOA quantum well comprising Indium,Gallium, and Arsenic.
 8. The wavelength tunable light source of claim 6,wherein a thickness of said at least one SOA quantum well is in a rangeof about 5-10 nm.
 9. The wavelength tunable light source of claim 6,wherein said semiconductor optical amplifier contains exactly 2 SOAquantum wells with at least one tensile-strained barrier.
 10. Thewavelength tunable light source of claim 6, wherein said at least oneSOA quantum well comprises two confined quantum states.
 11. Thewavelength tunable light source of claim 6, wherein a vertical beamdivergence of said semiconductor optical amplifier is less than about 25degrees full-width at half-maximum.
 12. The wavelength tunable lightsource of claim 6, wherein said first mirror comprises an alternatingstack including a first Al(x1)Ga(1−x1)As layer and a secondAl(x2)Ga(1−x2)As layer, where x1 and x2 are in a range of 0-1.
 13. Thewavelength tunable light source of claim 6, wherein said first mirrorcomprises an alternating stack comprising including a first materialcontaining aluminum and oxygen, and a second material that isAl(x)Ga(1−x)As, where x is in a range of 0-1.
 14. A wavelength-tunablelight source operative to emit wavelength tunable radiation over awavelength range contained in a wavelength span between about 950 nm andabout 1150 nm, said wavelength tunable light source comprising avertical cavity laser (VCL), said VCL having a gain region with at leastone compressively strained quantum well containing Indium, Gallium, andArsenic, said vertical cavity laser further comprising a first portionincluding a first mirror, a second portion including a second mirrorattached to a mechanical structure including a flexible membrane with asupport structure, an adjustable airgap between said second portion andsaid first portion, a first means for injecting electrons and holes intosaid gain region, a second means for adjusting said airgap, and a vacuumenvironment surrounding said vertical cavity laser.
 15. The wavelengthtunable light source of claim 14, wherein said vacuum environment isprovided by an evacuated butterfly package.
 16. The wavelength tunablelight source of claim 14, wherein said vacuum environment is provided byan evacuated transistor outline (TO) package.
 17. The wavelength tunablelight source of claim 14, wherein said wavelength range is repetitivelyscanned at a mechanical resonant frequency of said mechanical structure.18. The wavelength tunable light source of claim 14, wherein saidwavelength range is covered with a maximum voltage less than about 10V.19. A wavelength-tunable light source operative to emit wavelengthtunable radiation over a wavelength range contained in a wavelength spanbetween about 950 nm and about 1150 nm, said wavelength tunable lightsource comprising a vertical cavity laser (VCL), said VCL having a gainregion with at least one compressively strained quantum well containingIndium, Gallium, and Arsenic, said vertical cavity laser furthercomprising a first portion including a first mirror, a second portionincluding a second mirror attached to a mechanical structure including aflexible membrane with a support structure, an adjustable airgap betweensaid second portion and said first portion, a first means for electricalinjection of electrons and holes into said gain region, said first meansincluding a tunnel junction, a second means for adjusting said airgap,and a third means for obtaining substantially single longitudinal andtransverse mode operation over said wavelength tuning range.
 20. Thewavelength tunable light source of claim 19, further comprising a fourthmeans for constricting electrical current injection to an aperture. 21.The wavelength tunable light source of claim 19, wherein said fourthmeans comprises an oxidized aperture.
 22. The wavelength tunable lightsource of claim 19, wherein said fourth means comprises a buried tunneljunction.
 23. The wavelength tunable light source of claim 19, furthercomprising an n-type current spreading layer above said currentaperture.
 24. The wavelength tunable light source of claim 19, whereinsaid tunnel junction is substantially aligned with a minimum in astanding wave profile in said VCL cavity.
 25. The wavelength tunablelight source of claim 21, wherein said oxidized aperture issubstantially aligned with a minimum in a standing wave profile in saidVCL.
 26. The wavelength tunable light source of claim 19, wherein saidfirst mirror comprises an alternating stack including a firstAl(x1)Ga(1−x1)As layer and a second Al(x2)Ga(1−x2)As layer, where x1 andx2 are in a range of 0-1.
 27. The wavelength tunable light source ofclaim 19, where said first mirror comprises an alternating stackincluding a first material containing aluminum and oxygen, and a secondmaterial that is Al(x)Ga(1−x)As, where x is in a range of 0-1.
 28. Thewavelength tunable light source of claim 19, wherein electronic chargetravels around at least one insulating layer comprising aluminum andoxygen to a substrate contact.
 29. The wavelength tunable light sourceof claim 19, comprising exactly 3 compressively strained quantum wellsincluding Indium, Gallium, and Arsenic, and at least onetensile-strained barrier comprising Gallium, Arsenic, and Phosphorus.30. The wavelength tunable light source of claim 1, wherein said VCL isdriven by a periodic tuning waveform periodically adjusting said airgap,such that said wavelength tunable radiation has a periodic wavelengthvariation with time at a first wavelength repetition frequency and afirst repetition period, further comprising at least one optical delayline for generating at least one time-delayed copy of said wavelengthtunable radiation, a combiner for combining all of said time-delayedcopies into a common optical path to create a multiplexedwavelength-swept radiation, and a fifth means for turning off said VCLduring a time window of said first wavelength repetition period, whereinsaid multiplexed wavelength swept radiation has a second wavelengthrepetition frequency which is an integer multiple of said firstwavelength repetition frequency.
 31. The wavelength tunable light sourceof claim 30, wherein said fifth means comprises varying a drive currentin an electrically pumped VCL.
 32. A system for optical coherencetomography, the system comprising at least one wavelength tunable lightsource of claim 1, wherein said VCL is driven by a periodic tuningwaveform which periodically adjusts said airgap such that saidwavelength-tunable radiation is repetitively tuned over said wavelengthrange, a splitter for splitting said wavelength tunable radiation to asample and a reference path, an optical detector for detecting aninterference signal between a reflection from said sample and lighttraversing said reference path, and a signal processing system forconstructing an image from said interference signal.
 33. The system ofclaim 32, wherein said sample is an in-vivo human eye.
 34. The system ofclaim 33, wherein said image includes a portion of both the anterior eyeand the retina.
 35. The system of claim 32, wherein a dynamic coherencelength of said wavelength-swept radiation exceeds 100 mm.
 36. A systemfor optical coherence tomography, the system comprising at least onewavelength tunable light source of claim 6, wherein said VCL is drivenby a periodic tuning waveform which periodically adjusts said airgapsuch that said wavelength-tunable radiation is repetitively tuned oversaid wavelength range, a splitter for splitting said wavelength tunableradiation to a sample and a reference path, an optical detector fordetecting an interference signal between a reflection from said sampleand light traversing said reference path, and a signal processing systemfor constructing an image from said interference signal.
 37. The systemof claim 36, wherein said sample is an in-vivo human eye.
 38. The systemof claim 37, wherein said image includes a portion of both the anterioreye and the retina.
 39. The system of claim 36, wherein a dynamiccoherence length of said wavelength-swept radiation exceeds 100 mm. 40.A system for optical coherence tomography, the system comprising atleast one wavelength tunable light source of claim 14, wherein said VCLis driven by a periodic tuning waveform which periodically adjusts saidairgap such that said wavelength-tunable radiation is repetitively tunedover said wavelength range, a splitter for splitting said wavelengthtunable radiation to a sample and a reference path, an optical detectorfor detecting an interference signal between a reflection from saidsample and light traversing said reference path, and a signal processingsystem for constructing an image from said interference signal.
 41. Thesystem of claim 40, wherein said sample is an in-vivo human eye.
 42. Thesystem of claim 41, wherein said image includes a portion of both theanterior eye and the retina.
 43. The system of claim 40, wherein adynamic coherence length of said wavelength-swept radiation exceeds 100mm.
 44. A system for optical coherence tomography, the system comprisingat least one wavelength tunable light source of claim 19, wherein saidVCL is driven by a periodic tuning waveform which periodically adjustssaid airgap such that said wavelength-tunable radiation is repetitivelytuned over said wavelength range, a splitter for splitting saidwavelength tunable radiation to a sample and a reference path, anoptical detector for detecting an interference signal between areflection from said sample and light traversing said reference path,and a signal processing system for constructing an image from saidinterference signal.
 45. The system of claim 44, wherein said sample isan in-vivo human eye.
 46. The system of claim 45, wherein said imageincludes a portion of both the anterior eye and the retina.
 47. Thesystem of claim 44, wherein a dynamic coherence length of saidwavelength-swept radiation exceeds 100 mm.